1. Field of the Invention
The present invention relates to a flexible printed circuit board used for a head suspension assembly of a magnetic disk apparatus. More specifically, the present invention relates to a flexible printed circuit board capable of sufficiently preventing electrostatic discharge of a magnetic head even when a head suspension assembly is configured using a magnetic head with high performance.
2. Description of the Related Art
A magnetic disk apparatus widely used as a storage apparatus for a computer generally includes, as main components, a magnetic disk that is a recording medium, a spindle motor for rotating the magnetic disk, a head suspension assembly including a magnetic head, a carriage arm for supporting the head suspension assembly, a servo apparatus for moving the carriage arm, and a substrate unit for processing an electric signal. The head suspension assembly is composed of a magnetic head for recording/reproducing information, a suspension for placing the magnetic head, and a flexible printed circuit board connected to the suspension at one end and connected to the substrate unit at the other end. In this case, generally, the magnetic head is relayed to a connecting portion of a base end of the carriage arm by connecting the magnetic head to a side end of the suspension of the flexible printed circuit board through fixed wiring with a constitution in which a copper foil layer is interposed between polyimide layers, and connecting the side end of the suspension of the flexible printed circuit board to the substrate unit through the flexible printed circuit board.
The head suspension assembly with the above-mentioned constitution has a problem of ESD (electrostatic discharge) of the magnetic head. In particular, the flexible printed circuit board is covered with an insulating material such as plastic resin for the purpose of insulating a conductive circuit formed therein. Therefore, the flexible printed circuit board is likely to be charged by applying friction generated during handling. When the magnetic head is attached to the charged flexible printed circuit board, sudden discharge directed to the magnetic head occurs to cause electrostatic discharge thereof. Examples of the electrostatic discharge include the breakdown (hard ESD) of an insulating film of the magnetic head and the deterioration (soft ESD) in performance of the magnetic head. Both of them are not preferable, and become a factor for decreasing a production yield of the head suspension assembly. Furthermore, recently, along with an increase in recording density of the magnetic disk apparatus, there is a demand for high performance of the magnetic head, in particular, an increase in a reproduction output. At present, an MR (magnetoresistive) head and a GMR (giant magnetoresistive) head are developed in accordance with the demand, and furthermore, a GMR head utilizing a specular spinvalve film capable of generating a larger reproduction output is also proposed. These high-performance magnetic heads use a very thin insulating film. Therefore, they are very sensitive to electrostatic discharge (breakdown).
However, in a conventional head suspension assembly, a magnetic head is relayed to a substrate unit with two members. Therefore, it is required to connect fixed wiring to a conductive circuit in a flexible printed circuit board by soldering or the like in the course of manufacturing of the head suspension assembly. The static electricity generated during such a connection operation may cause electrostatic discharge of an insulating film of the magnetic head.
On the other hand, an attempt is made to allow a flexible printed circuit board to have conductivity so as to prevent electrostatic discharge of a magnetic head (see JP 11-250434 A). This attempt is characterized in that the flexible printed circuit board connecting the magnetic head to the substrate unit is composed of a base layer, a conductive pattern formed on the base layer, a cover layer covering the conductive pattern, and a conductive layer formed on the cover layer. By adopting the flexible printed circuit board with such a structure, static electricity charged by friction is discharge through the conductive layer to prevent electrostatic discharge.
Furthermore, in the flexible printed circuit board, it is generally required to electrically connect four connecting terminals of a magnetic head to conductive circuits in the flexible printed circuit board with sufficient bonding strength. An example of the connection method includes bonding balls made of metal to the connecting terminals and the conductive circuits through an ultrasonic wave. According to this method, an attempt is made to form convex portions projecting in a direction connecting the connecting terminals to the conductive circuits on the surfaces of the balls, thereby preventing the rotation of the balls during connection thereof, thereby enhancing connection strength and reliability (see JP 11-250433 A).
However, even in the flexible printed circuit board with the above constitution, a plurality of conductive circuits are exposed at the magnetic head connecting portion so as to connect the connecting terminals of the magnetic head to the conductive circuits in the flexible printed circuit board. A conductive layer cannot be easily formed on the exposed conductive circuits since the conductive layer may cause a short circuit between the individual conductive circuits. When the conductive layer is formed, it prevents not only electrical connection between the magnetic head and the conductive circuits, but also mechanical connection (bonding). Furthermore, the following is considered: only the conductive circuits are masked so that the conductive layer is not formed on the surface of the conductive circuits but on the other portions. Since the conductive circuits have a very minute constitution, however, masking thereof is difficult.